| 1. |
T. Tsuru, K. Shimizu, M. Yamaguchi, M. Itakura, K. Ebihara, A. Bendo, K. Matsuda, H. Toda, Hydrogen-accelerated spontaneous microcracking in high-strength aluminium alloys, Scientific reports, 10.1038/s41598-020-58834-6, 10, 1, 2020.04, Aluminium alloys are re-evaluated as most feasible way to satisfy the industrial needs of light-weight structural materials. However, unlike conventional structural metals such as iron and titanium, aluminium does not have easily accessible secondary phases, which means that aluminium-based alloys cannot be strengthened by harnessing multiple phases. This leaves age hardening as the only feasible strengthening approach. Highly concentrated precipitates generated by age hardening generally play a dominant role in shaping the mechanical properties of aluminium alloys. In such precipitates, it is commonly believed that the coherent interface between the matrix and precipitate does not contribute to crack initiation and embrittlement. Here, we show that this is not the case. We report an unexpected spontaneous fracture process associated with hydrogen embrittlement. The origin of this quasi-cleavage fracture involves hydrogen partitioning, which we comprehensively investigate through experiment, theory and first-principles calculations. Despite completely coherent interface, we show that the aluminium–precipitate interface is a more preferable trap site than void, dislocation and grain boundary. The cohesivity of the interface deteriorates significantly with increasing occupancy, while hydrogen atoms are stably trapped up to an extremely high occupancy over the possible trap site. Our insights indicate that controlling the hydrogen distribution plays a key role to design further high-strength and high-toughness aluminium alloys.. |
| 2. |
H. Su, H. Toda, K. Shimizu, K. Uesugi, A. Takeuchi, Y. Watanabe, Assessment of hydrogen embrittlement via image-based techniques in Al–Zn–Mg–Cu aluminum alloys, Acta Materialia, 10.1016/j.actamat.2019.06.056, 176, 96-108, 2019.09, Hydrogen repartitioning and the related embrittlement behavior were characterized by studying Al–Zn–Mg–Cu aluminum alloys with different intermetallic particle contents. Using high-resolution X-ray tomography and related microstructural tracking techniques, hydrogen-induced quasi-cleavage cracks and the related strain localization were observed regardless of the content of the intermetallic particles. The area of quasi-cleavage cracks on the fracture surface increased and the strain localization became more intense with a decrease in the content of intermetallic particles, thereby revealing that trapped hydrogen at intermetallic particles increases the resistance to hydrogen embrittlement. In addition, a quantitative assessment of the hydrogen repartitioning taking into account vacancy production and dislocation multiplication during deformation, was applied to characterize the hydrogen embrittlement behavior. Because of the thermal equilibrium among various hydrogen trap sites, internal hydrogen atoms are mainly repartitioned to vacancies and precipitates in the strain localization region during deformation because of their high trap site densities and high hydrogen trap binding energies. Since the concentration of hydrogen trapped at dislocations is extremely limited, it can be assumed that hydrogen repartitioned to precipitates induces decohesion of precipitates along specific crystallographic planes, where quasi-cleavage cracking may originate.. |
| 3. |
H. Su, H. Toda, R. Masunaga, K. Shimizu, H. Gao, K. Sasaki, Md S. Bhuiyan, K. Uesugi, A. Takeuchi, Y. Watanabe, Influence of hydrogen on strain localization and fracture behavior in Al–Zn–Mg–Cu aluminum alloys, Acta Materialia, 10.1016/j.actamat.2018.08.024, 159, 332-343, 2018.10, Hydrogen-induced dislocation motion is characterized in terms of the microscopic strain distribution in Al–Zn–Mg–Cu aluminum alloys. Hydrogen-induced strain localization was visualized in 3D using X-ray tomography and related microstructural tracking techniques. The strain localization was observed as a form of obliquely aligned shear bands. The strain localization becomes more intense with an increase in holding time at each loading step, indicating that more internal hydrogen is partitioned to the strain localization regions with holding time. In addition, the concentration of hydrostatic strain is observed in the strain localization region. Numerous nano voids were generated after deformation and were determined from the precise interpretation of the measured hydrostatic tension. Direct observation of the nano voids was then successfully performed by employing high-angle annular dark-field (i.e., HAADF) scanning transmission electron microscopy imaging and imaging-type computed tomography (CT) techniques. It is assumed that nano voids can serve a dual role as a fracture origin site and a hydrogen trap site. However, no evidence for hydrogen embrittlement originating from nano voids was observed. Instead, it can be assumed that the most hydrogen was partitioned to nano voids in strain localization regions during deformation due to its high density. A hydrogen embrittlement model was proposed based on these findings, where in-situ hydrogen repartitioning, which is necessary for hydrogen embrittlement to occur, is considered.. |
| 4. |
H. Toda, A. Takijiri, M. Azuma, S. Yabu, K. Hayashi, S. Dowon, M. Kobayashi, K. Hirayama, A. Takeuchi, K. Uesugi, Damage micromechanisms in dual-phase steel investigated with combined phase- and absorption-contrast tomography, ACTA MATERIALIA, 10.1016/j.actamat.2017.01.010, 126, 401-412, 2017.03, SPring-8を利用したX線CTを用いれば、結晶粒界にある粒子が鮮明に3D観察できることに着目した。このような粒子は、金属が変形し、破壊していく過程でも常に結晶粒界に位置する為、結晶粒界の粒子の情報を使えば、個々の結晶の形を4Dで求めることができると発想した。3Dイメージング法にX線回折法を組み合わせることで、多結晶組織の結晶学的変形を解析できる手法として完成した。また、この手法を実用的な構造材料に適用できるレベルにまで高精度化することに成功した。. |
| 5. |
H. Toda, T. Kamiko, Y. Tanabe, M. Kobayashi, D.J. Leclere, K. Uesugi, A. Takeuchi, K. Hirayama, Diffraction-amalgamated grain boundary tracking for mapping 3D crystallographic orientation and strain fields during plastic deformation, ACTA MATERIALIA, 10.1016/j.actamat.2016.01.072, 107, 310-324, 2016.04, SPring-8を利用したX線CTを用いれば、結晶粒界にある粒子が鮮明に3D観察できることに着目した。このような粒子は、金属が変形し、破壊していく過程でも常に結晶粒界に位置する為、結晶粒界の粒子の情報を使えば、個々の結晶の形を4Dで求めることができると発想した。3Dイメージング法にX線回折法を組み合わせることで、多結晶組織の結晶学的変形を解析できる手法として完成した。また、この手法を実用的な構造材料に適用できるレベルにまで高精度化することに成功した。. |
| 6. |
H. Li, H. Toda, K. Uesugi, A. Takeuchi, Y. Suzuki, M. Kobayashi, Application of diffraction-amalgamated grain boundary tracking to fatigue crack propagation behavior in high strength aluminum alloy, Materials Transactions, 10.2320/matertrans.M2014340, 56, 3, 424-428, 2015.03, Fatigue crack propagation behavior in a 7075-type Al alloy (Al-5.6%Zn-2.5%Mg-1.6%Cu) was investigated by applying Diffraction-Amalgamated Grain Boundary Tracking (DAGT), which provides grain morphologies and crystallographic orientations in three dimensions (3D). 3D crack morphologies at different propagation stages in the bulk of the sample were successfully obtained using synchrotron radiation X-ray microtomography (SRCT) technique. The apparent crack growth rate, da/dN, which varies significantly along a crack front line, was calculated. There are rapid acceleration and deceleration regions of da/dN due to the interaction with grain boundary (GB) and crack closure segments. Typical crack morphology under the influence of Mode II (in-plane shear) and Mode III (out-of-plane shear), such as crack deflection and twist, is detected by the observation of 2D tomographic slice image. A detailed direct assessment of microstructure-crack interaction behaviors has been achieved by applying the DAGT technique.. |
| 7. |
H. Toda, P. C. Qu, S. Ito, K. Shimizu, K. Uesugi, A. Takeuchi, Y. Suzuki, M. Kobayashi, Formation behaviour of blister in cast aluminium alloy, International Journal of Cast Metals Research, 27, 6, 369-377, 2014.06. |
| 8. |
H. Toda, H. Oogo, K. Horikawa, K. Uesugi, A. Takeuchi, Y. Suzuki, M. Nakazawa, Y. Aoki, M. Kobayashi, The true origin of ductile fracture in aluminum alloys, Metallurgical and Materials Transactions A, 10.1007/s11661-013-2013-3, 45A, 2, 765-776, 2014.02, It has generally been assumed that metals usually fail as a result of microvoid nucleation induced by particle fracture. Here, we concentrate on high-density micropores filled with hydrogen in aluminum, existence of which has been largely overlooked until quite recently. These micropores exhibit premature growth under external loading, thereby inducing ductile fracture, whereas the particle fracture mechanism operates only incidentally. Conclusive evidence of a micropore mechanism is provided by the observation of an instantaneous release of gas at failure. We can therefore conclude that the growth of micropores dominates ductile fracture. Since the material we used has a standard pore density, we can assume that an identical fracture mechanism operates in other aluminum alloys. This finding suggests that intense heat treatment, which is generally believed to enhance the mechanical properties through homogenization, may have entirely the opposite effect. This revelation will have a major impact on the engineering design of metals.. |
| 9. |
C. Gupta, H. Toda, T, Fujioka, M. Kobayashi, K. Uesugi, A. Takeuchi, Y. Suzuki, Micro-pore development phenomenon in hydrogen pre-charged aluminum alloy studied using synchrotron X-ray micro-tomography, Applied Physics Letters, 103, 17, 171902, 2013.10. |
| 10. |
H. Toda, Y. Ohkawa, T. Kamiko, T. Naganuma, K. Uesugi, A. Takeuchi, Y. Suzuki, M. Kobayashi, Grain boundary tracking technique: four-dimensional visualisation technique for determining grain boundary geometry with local strain mapping, Acta Materialia, 61, 14, 5535-5548, 2013.08, SPring-8を利用したX線CTを用いれば、結晶粒界にある粒子が鮮明に3D観察できることに着目した。このような粒子は、金属が変形し、破壊していく過程でも常に結晶粒界に位置する為、結晶粒界の粒子の情報を使えば、個々の結晶の形を4Dで求めることができると発想した。2011年度には、この手法を実用的な構造材料に適用できるレベルにまで高精度化することに成功した。この手法により、多結晶材料中のすべての結晶粒の形状の変化を時間を遡りながら4Dで観察することができるようになった。この成果は、金属材料工学では最も権威のある英文誌であるアクタ・マテリアリア誌に掲載が決定された。. |
| 11. |
H. Toda, K. Tsubone, K. Shimizu, K. Uesugi, A. Takeuchi, Y. Suzuki, M. Nakazawa, Y. Aokic, M. Kobayashi, Compression and recovery micro-mechanisms in flexible graphite, Carbon, 59, 184-191, 2013.08. |
| 12. |
H. Toda, Z. A. B. Shamsudin, K. Shimizu, K. Uesugi, A.Takeuchi, Y. Suzuki, M. Nakazawa, Y. Aoki, M. Kobayashi, Cavitation during high-temperature deformation in Al-Mg alloys, Acta Materialia, 10.1016/j.actamat.2013.01.012, 61, 7, 2403-2413, 2013.04, 過飽和な水素がアルミニウム内に分子状水素として排出され、ポアとして知られるミク ロ欠陥が生成されることに着目した。これらが常温の変形・破壊を支配することは、我々の最近の研究によって明らかにされている。従来、高 温での変形や加工では、粒界三重点でポアが発生して損傷に至るとされている。この研究では、高温変形の3D/4Dその場観察を行い、破壊起点を特定した。その結果、高温での変形や加工では、過飽和 な水素に起因する内在ポアの成長と従来の損傷機構（粒界三重点などでのポア発生）が競合し、前者が破壊を規定することが示された。. |
| 13. |
D. Seo, F. Tomizato, H. Toda, K. Uesugi, A. Takeuchi, Y. Suzuki, M. Kobayashi, Spatial resolution of synchrotron x-ray microtomography in high energy range: Effect of x-ray energy and sample-to-detector distance, Applied Physics Letters, 10.1063/1.4773239, 101, 26, 261901, 2012.12, これまで、鉄鋼材料のX線 による3D/4Dイメージングは、空間分解能が理論分解能より1～2桁低く、実用にはならなかった。この研究では、大型放射光施設SPring-8の活用と様々な撮像条件の最適化により、鉄鋼でもほぼ理論分解能での撮像が可能に なった。これは、主にはこれまで知られていなかったX線前方散乱による大きな悪影響を避けることで達成された。この研究の成果により、鉄鋼材料でも、熱処理や加工時の組織変化、変形・破壊・疲労破壊などの3D/4Dその場観察やその解析が可能になった。. |
| 14. |
H. Toda, S. Masuda, R. Batres, M. Kobayashi, S. Aoyama, M. Onodera, R. Furusawa, K. Uesugi, A. Takeuchi, Y. Suzuki, Statistical assessment of fatigue crack initiation from sub-surface hydrogen micropores in high-quality die-cast aluminum, Acta Materialia, 10.1016/j.actamat.2011.04.049, 59, 12, 4990-4998, 2011.07. |
| 15. |
H. Toda, E. Maire, S. Yamauchi, H. Tsuruta, T. Hiramatsu, M. Kobayashi, In situ observation of ductile fracture using X-ray tomography technique, Acta Materialia, 10.1016/j.actamat.2010.11.065, 59, 5, 1995-2008, 2011.03. |
| 16. |
H. Toda, T. Yamaguchi, M. Nakawaza, Y. Aoki, K. Uesugi, Y. Suzuki, M. Kobayashi, Four-Dimensional Annihilation Behaviors of Micro Pores during Surface Cold Working, Materials Transactions, 10.2320/matertrans.M2010069, 51, 7, 1288-1295, 2010.07. |
| 17. |
H. Toda, T. Nishimura, K. Uesugi, Y. Suzuki, M. Kobayashi, Influence of high-temperature solution treatments on mechanical properties of an Al-Si-Cu aluminum alloy, Acta Materialia, 10.1016/j.actamat.2009.11.044, 58, 6, 2014-2025, 2010.04. |
| 18. |
H. Toda, K. Minami, K. Koyama, K. Ichitani, M. Kobayashi, K. Uesugi, Y. Suzuki, Healing behavior of preexisting hydrogen micropores in aluminum alloys during plastic deformation, Acta Materialia, 10.1016/j.actamat.2009.06.012, 57, 15, 4391-4403, 2009.09. |
| 19. |
H. Zhang, H. Toda, P.C. Qu, Y. Sakaguchi, M. Kobayashi, K. Uesugi, Y. Suzuki, Three-dimensional fatigue crack growth behavior in an aluminum alloy investigated with in situ high-resolution synchrotron X-ray microtomography, Acta Materialia, 10.1016/j.actamat.2009.03.036, 57, 11, 3287-3300, 2009.06. |
| 20. |
H. Toda, T. Hidaka, M. Kobayashi, K. Uesugi, A. Takeuchi, K. Horikawa, Growth behavior of hydrogen micropores in aluminum alloys during high-temperature exposure, Acta Materialia, 10.1016/j.actamat.2009.01.026, 57, 7, 2277-2290, 2009.04. |
| 21. |
L. Qian, H. Toda, K. Uesugi, M. Kobayashi, T. Kobayashi, Direct observation and image-based simulation of three-dimensional tortuous crack evolution inside opaque materials, Physical Review Letters, 100, 11, 115505, 2008.11. |
| 22. |
A. Weck, D.S. Wilkinson, E. Maire, H. Toda, Visualization by X-ray tomography of void growth and coalescence leading to fracture in model materials, Acta Materialia, 10.1016/j.actamat.2008.02.027, 56, 12, 2919-2928, 2008.07. |
| 23. |
M. Kobayashi, H. Toda, Y. Kawai, T. Ohgaki, K. Uesugi, D.S. Wilkinson, T. Kobayashi, Y. Aoki, M. Nakazawa, High-density three-dimensional mapping of internal strain by tracking microstructural features, Acta Materialia, 10.1016/j.actamat.2007.12.058, 56, 10, 2167-2181, 2008.06. |
| 24. |
L. Li, H. Toda, T. Ohgaki, M. Kobayashi, T. Kobayashi, K. Uesugi, Y. Suzuki, Wavelet-based local region-of-interest reconstruction for synchrotron radiation X-ray microtomography, Journal of Applied Physics, 102, 114908-1-9, 2007.01. |
| 25. |
H. Toda, K. Uesugi, A. Takeuchi, K. Minami, M. Kobayashi, T. Kobayashi, Three-dimensional observation of nanoscopic precipitates in an aluminum alloy by microtomography with Fresnel zone plate optics, Applied Physics Letters, 10.1063/1.2359288, 89, 14, 143112, 2006.10. |
| 26. |
T. Ohgaki, H. Toda, M. Kobayashi, K. Uesugi, M. Niinomi, T. Akahori, T. Kobayashi, K. Makii, Y. Aruga, In situ observations of compressive behaviour of aluminium foams by local tomography using high-resolution X-rays, Philosophical Magazine A, 10.1080/14786430600724454, 86, 28, 4417-4438, 2006.10. |
| 27. |
L. Qian, H. Toda, K. Uesugi, T. Kobayashi, T. Ohgaki, M. Kobayashi, Application of synchrotron x-ray microtomography to investigate ductile fracture in Al alloys, Applied Physics Letters, 10.1063/1.214208, 87, 24, 241907, 2005.12. |
| 28. |
Y.L. Zhou, M. Niinomi, T. Akahori, H. Fukui, H. Toda, Corrosion resistance and biocompatibility of Ti-Ta alloys for biomedical applications, Materials Science & Engineering. A, 10.1016/j.msea.2005.03.032, 398, 1-2, 28-36, 2005.05. |
| 29. |
H. Toda, J. Katano, T. Kobayashi, T. Akahori, M. Niinomi, Assessment of thermo-mechanical fatigue behaviors of cast Al-Si alloys by experiments and multi-step numerical simulation, Materials Transactions, 46, 1, 111-117, 2005.01. |
| 30. |
K.H. Khor, J.-Y. Buffiere, W. Ludwig, H. Toda, H.S. Ubhi, P.J. Gregson, I. Sinclair, In situ high resolution synchrotron x-ray tomography of fatigue crack closure micromechanisms, Journal of Physics: Condensed Matter, 10.1088/0953-8984/16/33/012, 16, 33, S3511-S3515, 2004.08. |
| 31. |
H. Toda, I. Sinclair, J.-Y. Buffiere, E. Maire, K.H. Khor, P. Gregson, T. Kobayashi, A 3D measurement procedure for internal local crack driving forces via synchrotron X-ray microtomography, Acta Materialia, 10.1016/j.actamat.2003.11.014, 52, 5, 1305-1317, 2004.03. |
| 32. |
H. Toda, I. Sinclair, J.-Y. Buffiere, E.Maire, T. Connolley, M. Joyce, K.H. Khor, P. Gregson, Assessment of fatigue crack closure phenomenon in damage tolerant aluminium alloy by in-situ high-resolution synchrotron X-ray microtomography, Philosophical Magazine A, 83, 21, 2429-2448, 2003.07. |
